1. Field of the Invention
The present invention is directed to a power tools with multi-stage tightening torque control, and more particularly to a clutch-less power tool including a DC motor for tightening fasteners to a desired tightness through a plurality of control stages.
2. Description of the Prior Art
Japanese patent examined publication [KOKOKU] No. 60-47071 published on Oct. 19, 1985 discloses a torquing power tool utilizes an electric motor for tightening a fastener through two tightening stages. A control is firstly made to apply a high voltage to the motor for driving the fastener at a high speed until the fastener is detected to be seated. Upon detection of the seated condition which is made by monitoring a critical increase in a field current flowing through the motor, the motor is temporarily stopped. Then, the motor is restarted with an increasing voltage up to a predetermined limit and is again stopped after an elapse of a predetermined time period in order to further tighten the fastener at a starting torque of the motor. Unfortunately, however, such voltage control will not compensate for variation in resistance which is most likely in the electric circuit of the motor due to the temperature variation of the motor during a continued use and therefore fail to accurately determine a desired final tightening. Consequently, it may be still necessary to incorporate a mechanical clutch limiting the tightening torque in order to tighten the fastener reliably to an accurately predetermined tightness.
Another prior power tool is disclosed in Japanese patent non-examined early publication (KOKAI) No. 2-100882 published on Apr. 12, 1990. The patent proposes a multi-stage torquing power tool utilizes an electric motor for tightening a fastener through a plurality of tightening stages in which the fastener is tightened at differently controlled torques. The power tool includes a distance sensor to monitor a gap distance between the fastener's head and a work surface into which the fastener is being tightened and detect a pre-seated condition when the gap distance reduced to a predetermined value as indicative of that the fastener is just before seated. A control is firstly made to apply a high voltage to the motor for driving the fastener at a high speed until the fastener is detected to advance to the pre-seated condition, after which a low voltage is applied to the motor so as to tighten the fastener at a low speed to its seated condition for reducing an impact at the seating. Upon detection of the seated condition, a reverse voltage is applied to stop the motor. Then, the motor is restarted with a gradually increasing voltage from a relatively low voltage to a predetermined limit in order to further tighten the fastener at a correspondingly increasing torque. The predetermined voltage limit is selected to define a tightening torque at which the fastener is tightened into the work surface past the seated condition. Because the power tool also relies on the voltage control for determination of the tightening torque, it will suffer from resistance variation in the electric circuit of the motor and fail to tighten the fastener accurately and reliably at a desired torque.